1. Field of the Invention
The present invention relates to a compound solar battery and manufacturing method thereof and, more specifically, to a multi-junction type compound solar battery and manufacturing method thereof
2. Description of the Background Art
A multi-junction type III-V group compound solar battery has been known as a solar battery having highest efficiency and most suitable for aerospace applications among solar batteries. An exemplary method of manufacturing such a multi-junction type III-V group compound solar battery will be described in the following.
First, referring to FIG. 34, a Ge substrate (or a GaAs substrate) 101 is used as a substrate. On a surface of the substrate 101 of Ge or the like, Ge is epitaxially grown and AsH3 or PH3 is added to cause thermal diffusion of As or P, so that a bottom cell BB including a pn-junction of Ge is formed.
On the bottom cell BB, GaAs is epitaxially grown, so that a middle cell MM including a pn-junction of GaAs is formed. On the middle cell MM, InGaP is epitaxially grown, so that a top cell TT including a pn-junction of InGaP is formed.
In this manner, a 3-junction type III-V group compound solar battery 110 having a cell body CC is formed, in which three pn-junctions of Ge/GaAs/InGaP are connected in series in this order from the lower side on Ge substrate 101.
A forbidden band width (band gap) of InGaP forming the top cell TT is about 1.7 to about 2.1 eV, a forbidden band width (band gap) of GaAs as the middle cell is about 1.3 to about 1.6 eV, and a forbidden band width (band gap) of Ge as the bottom cell is about 0.7 eV or lower.
Sunlight enters from the side of top cell TT (InGaP) and proceeds toward the bottom cell BB (Ge), while light of prescribed wavelength is absorbed in accordance with the band gap of each of the top cell TT, middle cell MM and bottom cell BB, to be converted to electric energy.
The value of the band gap (about 0.7 eV or lower) of Ge as the bottom cell is relatively small considering the function of converting optical energy to electric energy. Therefore, use of a material having band gap of about 0.9 to about 1.1 eV has been proposed, as a material having higher conversion efficiency.
Reference 1 (M. Tamura et al., “Threading dislocations in InxGa1-xAs/GaAs heterostructures”, J. Appl. Phys. 72(8), Oct. 15, 1992, p. 3398) proposes InGaAs as one such material. In a multi-junction type solar battery 110 using InGaAs in place of Ge (see FIG. 35), on Ge substrate (or GaAs substrate) 101, a bottom cell NN including a pn-junction of InGaAs is formed by epitaxial growth
On the bottom cell NN, the middle cell MM including the pn-junction of GaAs and the top cell TT including the pn-junction of InGaP would be formed by epitaxial growth, respectively.
Reference 2 (J. F. Geisz et al., “Photocurrent of 1 eV GaInNAs lattice-matched to GaAs”, J. Crystal Growth 195 (1998), p. 401) proposes, in addition to InGaAs, InGaAsN as a material to replace Ge.
Multi-junction type solar battery 110 having the bottom cell NN employing InGaAs or InGaAsN in place of Ge, however, has the following problems.
First, in a multi-junction type solar battery employing InGaAs (0.9 to 1.1 eV) as the bottom cell NN, lattice constant of Ge substrate (or GaAs substrate) 101 is different from that of InGaAs. Therefore, epitaxially grown InGaAs comes to have a dislocation derived from the difference in lattice constant from the underlying layer (GaAs substrate or the like) (hereinafter referred to as a “misfit dislocation”).
In the multi-junction type solar battery employing InGaAsN as the bottom cell, a composition of N atoms will be controlled such that the lattice constant of InGaAsN matches the lattice constant of the underlying layer. Therefore, generation of misfit dislocation can be prevented in the epitaxially grown InGaAsN.
It is noted, however, that there would be holes and the like of added N atoms themselves. As a result, the epitaxially grown InGaAsN comes to have defects derived from N atoms.
As described above, the bottom cell formed of InGaAs or InGaAsN suffers from generation of misfit dislocation or defects, and therefore it does not have satisfactory cell quality. Accordingly, desired electricity production cannot be attained.
Further, the misfit dislocation or defects in the bottom cell NN has undesirable influence on GaAs as the middle cell MM epitaxially formed on the bottom cell NN and on InGaP as the top cell TT further formed thereon.
Consequently, cell quality of GaAs and InGaP is also degraded, preventing improvement in efficiency of electric energy conversion.
As described above, sunlight enters from the top cell TT and proceeds to bottom cell BB while light of a prescribed wavelength is absorbed and converted to electric energy.
At this time, component of the sunlight that is not absorbed by the top cell TT to bottom cell BB is eventually absorbed by Ge substrate (or GaAs substrate) 101 and hence that component cannot effectively contribute to generation of power.
As a result, improvement in efficiency of electric energy conversion is prevented.